More than half of all U.S. teenagers who use mobile apps have avoided certain apps because they would have to share personal information, according to a new study from the Pew Internet Project.

The study, done in conjunction with Harvard University’s Berkman Center for Internet & Society, surveyed about 800 teens aged 12 to 17 last summer to examine how they use technology and deal with privacy issues. About 37% of all U.S. teens have smartphones and 23% own a tablet computer. Of those teens, 81% have downloaded apps.

What’s notable is that 51% of app users say they have avoided downloading an app because of privacy concerns. And 26% said they deleted an app after discovering it was collecting personal information.

Two examples that often came up when surveying teens were requests for access to photos and contacts, said Mary Madden, a senior researcher at Pew who authored the report.

The study is timely: LinkedIn earlier this week said it would extend its professional social network to people as young as 14 in the U.S., hoping to connect high-schoolers with universities and alumni. Jeffrey Chester, who heads the Center for Digital Democracy, a nonprofit consumer-advocacy group, says teenagers often consider phones an extension of themselves, and have reason to be careful. “There is no mobile privacy at all,” he said.

Older teens are more likely to have a device that uses apps than younger teens, but there is little difference when it comes to downloading apps (except that teens outpace adults in downloading). In focus-group discussions, Pew found that teens tend to download social-media apps and games, and generally stick to free apps.

Of teen app users, 46% said they turned off location-tracking features, either through the phone’s settings or inside an app, because they were worried about others accessing that data.

Location tracking powers everything from local weather to timely deals when walking by a store to getting help in finding a lost phone. The tracking technology, which is enabled by global positioning satellites, has been a hot-button issue for years because of privacy and safety concerns.

Girls are far more likely to disable location tracking than boys–59% to 37%–the study found. That’s partly the power of “stranger danger” messages from adults and in schools, Madden said, but also because girls are heavier users of social-media apps like Facebook, which often use location tracking. In some of these cases, mind you, the trackers that teens are trying to avoid are their own parents.

In Pew’s focus groups, some teens said they purposefully left tracking on in cases where it made sense, like mapping software. Some left all tracking enabled, either because they wanted an app’s full feature set or because they didn’t think they had enough data on a phone to piece together an identity.

Teens tend to think about privacy in the sense of “social privacy” or whether an app is “creepy,” Madden said, not in terms of advertising or governmental surveillance like adults do.